Spatial and Temporal Changes in the Amphibian Egg

Early development in vertebrates encompasses a period of rapid cell division and regional differentiation. The egg goes from a single radially symmetric cell, arrested in its cell cycle, to an embryo composed of many rapidly proliferating cells organized spatially along lines that presage the overall topography of the adult. During this time, a pattern of division must be imposed on the individual blastomeres, and areas of the embryo that must be delineated will develop into specific tissues. In this review, we will consider some of the events immediately following fertilization that are important to the development of the amphibian embryo. We will be concerned with problems of organization, both spatial and temporal, and specifically with the mechanisms that regulate the timing of the cell cycle and determine the dorsal-ventral axis of the embryo. The unfertilized Xenopus egg is arrested in second meiotic metaphase, where no DNA synthesis is occurring (see review by Gerhart 1980). After fertilization, the egg undergoes a series of rapid divisions lasting about 25 minutes, with alternate periods of DNA synthesis and mitosis. During this time, the cell cycle appears to be a rudimentary one, having only a mitotic phase and a DNA synthesis phase, with little if any RNA synthesis (Bacharova and Davidson 1966, Brown and Littna 1964, Graham and Morgan 1966). The cleavages during this period are metachronous (Hara 1977,

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